1. Field of the Invention
The present invention relates to a process for the preparation of N-alkyl sulfenimides. More particularly, the present invention relates to a novel process for converting N-alkyl sulfenamides to N-alkyl sulfenimides.
2. Description of Related Art
Sulfenimides have been used commercially as vulcanization accelerators and are said to be useful as pesticides. Sulfenimides have been made in the past by placing an N-alkyl sulfenamide in an organic solvent and converting the sulfenamide to the sulfenimide by the addition of anhydrous HCl gas. In this case, the amine hydrochloride salt is formed as a by-product. The amine hydrochloride salt is then removed by filtration or extraction and the product is isolated by crystallization and filtration. Alternatively, sulfenimides can be made from N-alkyl sulfenamides by placing the N-alkyl sulfenamide in an anhydrous organic solvent and adding an organic acid anhydride. This reaction produces the desired sulfenimide along with the free acid and the N-alkyl amide corresponding to the N-alkyl group of the starting sulfenamide and the acid anhydride used. Sulfenimides have also been made by the reaction of a sulfenyl chloride with an amine in an anhydrous solvent.
Reactions of sulfenamides with carbonyl compounds have been reported in the past. However, none of the these reactions has been found to produce a sulfenimide as the major product of the reaction.
U.S. Pat. No. 2,860,142 discloses a process for the conversion of sulfenamides to sulfenimides comprising treating an N-alkyl or N-cycloalkyl benzothiazolyl-2-sulfenamide with acetic anhydride or a homolog thereof, with or without the presence of a second solvent, at about 25 to about 75.degree. C., for an appropriate period that can range from about 10 minutes to 24 hours. The resulting sulfenimide is isolated by crystallization, dilution, or by stripping off the solvent. The yields are very good.
U.S. Pat. No. 3,151,122 discloses a process for the preparation of N-alkyl-and N-cycloalkylbis(2-benzothiazolyl)sulfenimides comprising treating the corresponding 2-benzothiazolylsulfenamide under substantially anhydrous conditions with an acid having an ionization constant K.sub.a above 1.times.10.sup.-3 at 25.degree. C. The compounds are useful as accelerators in the vulcanization of any of the recognized sulfur-vulcanizable rubbers and show good resistance to scorch.
U.S. Pat. Nos. 5,079,305 and 5,189,174 disclose heterocyclic thiol sulfenimide compounds that are said to be useful as accelerators in the curing of natural and/or synthetic rubbers. The compounds can be represented by the formula: ##STR2##
Herein each of A.sup.1 and A.sup.2, independently, is at least one heterocyclic ring containing one or more nitrogen atoms therein, or at least one heterocyclic ring containing one or more nitrogen atoms and (1) one or more hydrocarbyl substituents; or (2) one or more electron withdrawing groups; or (3) one or more electron releasing groups, wherein A.sup.1 alternatively is thiazyl or a dithiocarbamoyl, and wherein R is: (a) a hydrogen atom, or (b) a hydrocarbyl radical having from 1 to 16 carbon atoms, or (c) the hydrocarbon radical of (b) wherein one or more of the carbon atoms is an oxygen atom, a nitrogen atom, or a substituted nitrogen atom wherein the substituted group is an alkyl and, in combination with the nitrogen atom, forms a heterocyclic ring having a total of one to seven carbon atoms, or (d) the hydrocarbyl radical of (b) containing a halogen, an amino, a cyano, an alkoxy, a hydroxy, or an alkoxycarbonyl. When the heterocyclic thiol sulfenimide compound is utilized as an accelerator for rubber, improved cure rates, longer scorch delay, and better reversion resistance are said to be obtained in comparison to conventional sulfenamides.
U.S. Pat. No. 5,204,481 discloses a process for producing N-alkyl or N-cycloalkyl-2-benzothiazolyl sulfenimides wherein aliphatic hydrocarbons are used as a reaction medium in converting N-alkyl or N-cycloalkyl-2-benzothiazolyl sulfenamides to the corresponding sulfenimides by reaction with an acid.
Ignatov et al., Zhurnal Obshchei Khimii, 47(5):1096-1103 (1977) studied the reactions of 2-benzothiazolylsulfeneamide and N-cyclohexyl-2-benzothiazolylsulfenamide with acetic acid, acetyl chloride, acetic anhydride, benzoyl chloride, picryl chloride, butyl acetate, maleic anhydride, phthalic anhydride, hydrochloric acid, trichloroacetic acid, thioacetic acid, and hydrogen sulfide. This enabled the authors to determine the optimum conditions for the synthesis of bis-2-benzothiazolylsulfenimide and N-cyclohexyl bis(2-benzothiazolyl)sulfenimide, which compounds are said to have valuable properties as general purpose rubber vulcanization accelerators.